Degree of difficulty 2 (of 5)

Minimum aperture Naked eye

Designation NGC 6254

Type Globular cluster

Class VII

Distance 24,750 ly (Hipparcos, 2001)

Size 140 ly

Constellation Ophiuchus

R.A. 16h 57.1min

Decl. −4° 6′

Magnitude 6.6

Surface brightness –

Apparent diameter 19′

Discoverer Messier, 1764

History Charles Messier discovered M 10 on the 29th of May 1764, just a night after its fainter neighbor M 9, and a night before he found its close neighbor M 12. He observed “a nebula without star” and added: “This nebula is beautiful & round; one can see it only with difficulty in a simple refractor of 3 ft. 4′ diameter.”

20 years later, William Herschel's telescope showed “A very beautiful, and extremely compressed, cluster of stars” and no trace of nebulosity. Herschel compared M 10 with M 53. His son John wrote in 1831: “A superb cluster of very compressed stars, gradually brighter toward the middle. The stars are of 10th to 15th magnitude, and run up to a blaze in the center, but I see no nucleus. Diameter about 6′; a noble object.”

Admiral Smyth used a 5.9-inch refractor and saw a “rich globular cluster of compressed stars. This noble phenomenon is of a lucid white tint, somewhat attenuated at the margin, and clustering to a blaze in the center. Easily resolved with medium magni. cation.” Lord Rosse reported, after his observation with his giant telescope: “A dark lane above the center quite across, or rather the upper one-sixth of cluster is much fainter than the rest.”

The description of Curtis, based on his photographs, says: “Fine bright globular cluster, diameter about 8'. Central brighter part about 2′.”

Astrophysics With a distance of 24,750 lightyears, M 10 is a little farther away than its close neighbor M 12, and its true diameter of 140 light-years is larger than the latter's.

With about 250,000 solar masses, M 10 is a rather average globular cluster.